1. Field of the Invention
This invention relates to a method for the production of zinc acrylate by the reaction of acrylic acid and zinc oxide by virtue of the use of a specific mixed solvent and more particularly to a method for the production of zinc acrylate which enables the crystal form of zinc acrylate to be adjusted owing to the use of the specific mixed solvent.
2. Description of the Related Art
Zinc acrylate is a useful compound as a cross-linking agent and is used as added in rubber composition to improve the vulcanizability or used as a modifier for a varying synthetic resin.
As a means to obtain zinc acrylate, a method which comprises inducing acrylic acid to react with a zinc compound in an organic solvent, distilling the reaction solution to expel the organic solvent, and thereafter drying the residue of the distillation (JP-B-58-14416) and a method which comprises filtering the reaction solution to remove the organic solvent and thereafter drying the residue of the filtration have been available. These methods, however, are deficient in workability because their reaction products stick tenaciously to the inner walls of their reaction vessels and to the agitating elements or aggregate into clusters. When the solvent is removed by decompression, the solvent contained in the produced zinc acrylate is scattered and the produced zinc acrylate is partly scattered as well to lower the yield thereof and the economy of the production is heavily impaired because the decomposition necessitates installation of extra devices for separation and recovery, for example.
For the sake of using zinc acrylate by kneading it in a rubber composition, numerous methods for coating the surface of zinc acrylate particles by the addition of a higher fatty acid such as stearic acid or a zinc salt thereof have been proposed (JP-A-52-154436, JP-A-53-83834, JP-60-94434, and JP-A-02-218639).
These methods, however, necessitate incorporation of a step for coating the surface of particles of zinc acrylate by the addition of such a high fatty acid as stearic acid or a zinc salt thereof besides a step for effecting the reaction of acrylic acid with a zinc compound and installation of devices appropriate for such steps. Further, for the purpose of enabling the produced zinc acrylate to be kneaded in a rubber composition, it is required to be an impalpable powder having a particle size of not more than 325 mesh (44 μm). Thus, these methods further necessitate a step for pulverizing the produced zinc acrylate into an impalpable powder. This step of pulverization not only calls for a huge labor but also entails the possibility of deteriorating the working atmosphere and inducing the problem of hygiene because zinc acrylate is unusually liable to emit dust during and after the work of pulverization.
A method which produces zinc acrylate by using toluene as a reaction solvent in the presence of an anionic surfactant, adding acrylic acid and a higher fatty acid of 12 to 30 carbon atoms thereto while dispersing zinc oxide therein, causing the zinc oxide to react with the acrylic acid, and adding the product of this reaction to the organic solvent has been also available. This method can produce fine zinc acrylate. The produced zinc acrylate, when kneaded in a rubber composition, very rarely induces cohesion and aggregation, disperses uniformly, and brings the kneading to completion (Official gazette of JP-A-09-202747).
When the higher fatty acid is reacted with acrylic acid in the toluene which has zinc oxide dispersed therein, the product aimed at must be dried after it has been separated from the organic solvent because the reaction generates water of constitution. The produced zinc acrylate is possibly aggregated during the course of this drying. For the process of producing zinc acrylate, the step of pulverization is indispensable because the presence of aggregated clusters results in degrading the dispersibility.
The samples of zinc acrylate produced in Examples 1–4 inserted in the official gazette of JP-A-09-202747 invariably contain particles exceeding 500 μm in diameter in proportions exceeding 20% by weight of the whole particles and, in their unaltered form, entail a disadvantage in failing to be smoothly kneaded or satisfactorily dispersed without producing aggregated clusters in rubber. They contain zinc acrylate particles of diameters not more than 5 μm in proportions in the range of 40–43% by weight while they contain the same particles of a diameter of not more than 44 μm in proportions in the range of 63–66% by weight. This fact indicates that the step of pulverization is indispensable to the production of fine particles of zinc acrylate possessing a fine and uniform particle diameter. Thus, the desirability of developing zinc acrylate which can be smoothly kneaded in a rubber composition and manifests satisfactory dispersibility thereto has been commanding strong general recognition. The methods known to the art, however, fall short of fulfilling the demand.
The produced zinc acrylate, for the sake of ensuring safe transportation and storage, is expected to avoid incurring repeated aggregation even during a protracted storage and, for the sake of facilitating transportation and storage, is expected to possess as high build density as permissible. The conventional products of zinc acrylate, however, are liable to incur aggregation during storage and this aggregation constitutes one cause for degrading the flowability of the compound at the time of use.
Regarding the process for the production of zinc acrylate, while the adhesion of the reaction product to the inner wall of the reaction vessel and to the agitation element mentioned above has entailed such problems as degrading the workability and jeopardizing the economy of the production, the problems have not been satisfactorily coped with.